Asteroid discovery suggests half of Earth’s water could have cosmic origin

2 May 2019

Illustration of an asteroid, not Itokawa. Image: © chagpg/

After examining the precious samples of asteroid material brought back by Japan, scientists have excitedly announced the discovery of water.

As the Japanese Aerospace Exploration Agency (JAXA) closely watches the progress of its Hayabusa 2 spacecraft to return samples of the asteroid Ryugu to Earth, its predecessor Hayabusa has helped uncover a major breakthrough in astrobiology.

Two cosmochemists from Arizona State University (ASU) have announced the first ever measurements of water from the surface of an asteroid, suggesting half of the Earth’s ocean water could have been delivered by similar objects.

In a paper published to Science Advances, the researchers detailed the discovery of the mineral pyroxene in two of the five particles recovered from the asteroid Itokawa. Here on Earth, pyroxenes have water in their chemical structure, leading Ziliang Jin and Maitrayee Bose to see if the asteroid samples also contained water, and if so how much.

More than 500 metres long and up to 300 metres wide, Itokawa is a peanut-shaped S-type asteroid. Among the most common type in an asteroid belt, this asteroid is actually the remnant of a larger asteroid almost 20km wide that suffered a number of large impacts. Two of the resulting fragments collided, resulting in its current shape approximately 8m years ago.

The samples obtained by Hayabusa came from a smooth and dust-covered area of the asteroid called the Muses Sea, possibly from as deep as 100 metres inside the object. But despite the catastrophic life of the space debris – including prolonged exposure to radiation – the recovered minerals showed that the water has not been lost.

Important discoveries

“This means S-type asteroids and the parent bodies of ordinary chondrites are likely a critical source of water and several other elements for the terrestrial planets,” said Bose.

“And we can say this only because of in situ isotopic measurements on returned samples of asteroid regolith – their surface dust and rocks. That makes these asteroids high-priority targets for exploration.”

She added that this will hopefully be one of many future discoveries found in asteroids, given the eventual return of Ryugu samples to be returned by Hayabusa 2 to Earth in December 2020.

“The Hayabusa mission to Itokawa has expanded our knowledge of the volatile contents of the bodies that helped form Earth,” Bose said. “It would not be surprising if a similar mechanism of water production is common for rocky exoplanets around other stars.”

Colm Gorey was a senior journalist with Silicon Republic